1. Field of the Invention
The present invention relates to a rotation control for a magnet used in a magneto-optical recording system and is directed more particularly to an apparatus for applying a magnetic field to an optical disc when an information is recorded on the optical disc.
2. Description of the Prior Art
An optical disc has a recording layer formed on its recording face. This recording layer is generally a magnetic thin film made of such as GdCo, FeTe, GdTbFe or the like. There is already proposed an apparatus by which information can be recorded on the recording layer of the optical disc by irradiating the layer with a laser beam and applying a magnetic field to the recording layer so that a magnetization formed in the recording layer in the direction perpendicular thereto is inverted by the Kerr effect.
Since such an optical recording disc is capable of recording data produced by a laser beam spot of several microns of meters much information can be recorded therein at a very high recording density, and thus it is useful as an external storage apparatus for a computer or the like. When data is recorded on or erased from the optical recording disc, a magnetic field of particular polarity must be applied to the recording layer of the optical recording disc.
FIG. 1 is a schematic representation of an apparatus used to apply a magnetic field to an optical recording disc.
Referring to FIG. 1, there is located on an optical disc 1 a recording layer having the magneto-optical effect. A permanent magnet 2 (hereinafter simply referred to as a magnet is provided to apply a magnetic field to the optical disc 1 at its surface. A coil 3 is provided to concentrically surround the magnet 2. When a current is supplied to the magnetic coil 3 in a particular direction, the magnet 2 rotates around a supporting shaft 4 to face its N pole or S pole to the surface of the optical disc 1. A magnetic member 5 is located at a position displaced from the center of the magnet 2. When the coil 3 is supplied with a current that inverts the magnet 2 in polarity, the magnet 2 is at first attracted to the magnetic member 5 and rotated in the counter-clockwise direction. Then, the magnet 2 is rotated by 180.degree. to exchange its N pole and S pole. An optical head 6 is provided to irradiate the recording surface of the optical disc 1 with a laser beam.
When information is recorded by such recording apparatus on the optical disc 1 (so-called orientation disc) in which its recording layer is magnetized in the same direction as shown in FIG. 1 by arrows, the power of the laser beam must be intensified and at the same time, the coil 3 must be supplied with a current opposite in direction to thereby rotate the magnet 2 by 180.degree., thus inverting the magnetic field applied to the surface of the optical disc 1. Then, if the optical disc 1 is rotated while the laser beam is being modulated in view of data to be recorded, the magnetization in the recording layer on the portion irradiated by the laser beam is inverted as represented by arrows in FIG. 2, thus making it possible to record the data in the recording layer during a time duration WT.
As described above, the conventional optical disc recording apparatus employs the coil 3 to invert the magnet 2 in polarity upon recording (or erasing) the data. In order to record or erase data more rapidly, the magnet 2 must be inverted in polarity stably and more quickly. Thus, the coil 3 must be supplied with a larger inverting current.
When the optical disc recording apparatus is placed in its recording mode, the coil 3 is supplied with a large start or kick signal P1 at time T1 as shown in FIG. 3A. Thereafter, the coil 3 is supplied with a current i that keeps the magnet 2 in its inverted state. At the completion of the recording, at time T2, a kick signal P2 of opposite polarity to the signal P1 is supplied to the coil 3 and then, the coil 3 is supplied with a current i that keeps the magnet 2 in its initial state. Although then the direction of the magnetic field to the surface of the optical disc 1 is inverted from the N pole to the S pole as shown in FIG. 3B, the magnet 2 is accelerated and decelerated at the completion of the inversion due to the existence of the magnetic member 5 and the attracting force caused by the holding current i supplied to the coil 3, so that until the magnet 2 is settled at a predetermined position a longer settling time ta is required.
As a result, it takes a lot of time for the optical disc recording apparatus to be set in the recording mode for the optical disc 1. Thus, when the optical disc 1 is employed as an external storage apparatus for the computer or the like, it takes a lot of operation time.